Switched mode or buck mode converter power supplies convert DC to DC voltages. These supplies are characterized by low output ripple voltage but generally do not provide DC isolation between the input and output return. When isolation is required it is provided by multiple secondary windings in a transformer. Accommodating the isolation requirement using a transformer requires additional secondary windings, typically one winding per individual output. In a large number of instances power is provided to multiple outputs, requiring as many secondary windings as there are outputs. Multiple isolated secondary windings add design time to achieve proper turns ratio per output with respect to the common primary winding. Also, designing internal coupling of the fields within a transformer so as to be made equal for all windings complicates the design effort. Multiple isolated secondary windings built into the main transformer generally require extra output connections and added pins on the transformer header for each isolated output. Additionally, a transformer characteristically has a large footprint and as the transformer must be made even larger to accommodate multiple windings, it results in a further depreciation of power supply density. Independent diode pairs, quads or synchronous rectifiers with drive circuits and also requiring filters are further needed for each isolated output. This also increases complexity, drives up transformer design cycle times and costs, and places the design at risk, especially if an additional supply output is required later in the design cycle. This point applies to transformer design cycle redesigns.
A less complex transformer or a substitution for the transformer in a buck-derived topology configuration would be desirable. In addition to a transformer substitution, less filtering circuitry would reduce the complexity of the topology and require fewer components, increasing reliability, increasing power efficiency and decreasing cost. Outputs having either voltage polarity and that also supply stepped up or down voltages while minimizing ripple current with respect to the input are desirable features in many power supply applications.
Often there exists the dual requirement of delivering power from the power supply while removing the heat it generates. In most instances the solution is to attach the power supply device, typically its active components, such as an FET switch or gate, to a heat sink. The heat sink is then referenced to ground via the power supply chassis. In some cases, the power supply, especially the active components must be insulated from the heat sink by a thin dielectric material. These materials, often in combination with the heat sink and the components attached thereto form a capacitor; that is, the heat sink operates as one of the plates of a capacitor with the power supply (e.g., an FET drain) device operating as the other plate. If the back of the power device, which for N-channel FETs is a drain then whenever the drain voltage transitions, typically in the timeframe of hundreds of nanoseconds, the current can virtually instantaneously reach millions of amperes as determined by the relationship, C (dv/dt). Generally, this current has no direct return to ground so it circulates within the chassis causing electrical noise. This circulation is referred to as an injected chassis current. A provision for re-routing the injected chassis currents lowers radiated emissions and lowers the noise injected into other circuits in close proximity. A device that eliminates the injected currents would reduce the radiated emissions and other noise generating effects.